The antennas of mobile communication devices, in particular, planar antennas, react to a change in their surroundings by a change in their impedance. The transmission coefficient is dependent on the impedance matching between a front end circuit of a mobile communication device and the antenna, the transmission coefficient describing in the case of transmission signals that fraction of the power from the front end circuit which is emitted via the antenna. At the same time, the transmission coefficient of reception signals which are received via the antenna and are forwarded to the front end circuit is also dependent on the degree of impedance matching. In order, for a given emitted power, to minimize the power to be expended for this in the front end circuit or in order to be able to obtain a good reception signal, it is correspondingly important for the front end circuit and the antenna to be impedance-matched to one another.
In previous TDD (Time Division Duplexing) methods, an adaptive matching circuit serves for dynamically matching the impedance of the front end circuit to the variable impedance of the antenna. In such a system, different frequency ranges are generally used for transmission signals and reception signals. Since the processes of transmission and reception take place alternately temporally in so-called time slots (for example, in the case of the GSM standard), however, impedance matching is only necessary for the frequency range currently in use.
The situation is different in the case of radio standards which enable simultaneous transmission and reception (FDD methods, Frequency Division Duplexing), such as the European W-CDMA standard, for example. Transmission signals are communicated in transmission frequency ranges (so-called “uplink” frequencies), whereas reception signals are communicated in so-called “downlink” frequencies. Such frequency ranges are additionally separated from one another by the so-called band gap arranged therebetween. Therefore, an FDD impedance matching circuit typically has to enable sufficient matching in a frequency range which simultaneously serves transmission and reception frequency ranges.
A further problem in relation to impedance matching circuits known from the prior art is that the present matching circuit is electrically connected between a duplexer and the antenna, whereas there is usually no duplexer provided in TDD systems. A duplexer is a necessary part of an FDD front end circuit which serves as a frequency-separating network and, owing to its own frequency-dependent impedance, makes it more difficult to achieve sufficient impedance matching.